The present invention relates to pump-off controllers for beam pumping systems used in producing oil wells. The term `beam pumping systems` refers to pumping units of the type having a walking beam for reciprocating a rod string that extends down the well to operate a pump unit located at the bottom of the well. The downhole pump has a travelling valve in the plunger and standing valve at the bottom of the pump barrel. The travelling valve opens on the downstroke when the plunger contacts fluid in the barrel and closes on the upstroke while the standing valve remains closed on the downstroke and opens on the upstroke to allow fluid to enter the barrel.
Pump-off controllers are used to shut down beam pumping systems when the well has pumped off, the controller re-starts the beam pumping system after a preset down time. The term "pumped-off" is used to describe the condition where the downhole pump does not completely fill with fluid on the upstroke of the pump. On the succeeding downstroke the rod string and plunger of the pump fall until the plunger contacts the fluid in the pump barrel. When the piston contacts the fluid, a vibration or shock wave is transmitted through the rod string to the beam pumping unit. This can cause damage and failure of the rod string or pumping unit. In addition, when the pump is not completely filled with fluid, the pump is not lifting as much fluid as when the pump is full. This can result in increased energy costs for the quantity of fluid produced.
U.S. Pat. No. 3,951,209 describes a pump-off detection method in which the load on the rod string and the position of the rod string are measured. From the load versus displacement measurements the energy input to the top of the rod can be calculated by integrating the product of load times displacement. When the well has pumped off, the energy input to the rod will be reduced since the load on the rod at the start of the downstroke remains high. The reduction in energy input to the rod string can be used as a control signal for controlling the operation of the pump unit.
U.S. Pat. No. 4,015,469 describes an improvement of the method described in the above patent wherein the energy input to the rod is calculated for only a portion of the pump stroke. As described in this patent, the greatest change in the energy input to the rod occurs during the first part of the downstroke of the pump. The greater change in the energy input produces a more reliable detection of when the well has pumped off.
U.S. Pat. No. 4,583,915 describes a method for detecting pump-off which calculates an area bounded by two positions of the rod string and the minimum rod load and the actual load. While this is not a true calculation of the energy input to the rod, it can be related to the area calculated in U.S. Pat. No. 4,015,469. The area that is measured in the U.S. Pat. No. 4,583,915 patent is outside the dynagraph or pump card while only the area inside the card represents the energy input to the rod.
A pump-off controller sold by Baker-CAC of Houston, Tex. and referred to a Baker Model 8500 utilizes percentages of the measured load and displacement to set limits for determining pump-off. This pump-off controller detects pump-off by tracking where the measured load crosses the set load line. When the crossing point moves to the left of the position line the well is pumped off. This controller does not monitor the energy input to the rod string as described in the above referenced patents.
The methods described in the above patents for determining pump-off are satisfactory in many applications but fail in some other applications. In the case of a high fluid level caused by the long shutdown of the pumping unit, the calculation of an area gives a false pump-off signal and prematurely shuts the pumping unit down. This, of course, reduces the total production from the well. Similar problems occur when gas is present in the well fluid.
In addition to the above problems, the prior systems, while including means for correcting the various devices used to measure load and position for various errors, did not provide an accurate result. For example, errors introduced by temperature changes or errors that result from incorrect data relating to fixed pump parameters. Likewise, errors can result from a failure to properly calibrate the measuring devices used to measure the load on the rod string and the position of the rod string.